Electrical melting is known in glass furnaces where heat is generated by the Joule effect caused by electrical current flow between pairs of electrodes submerged in molten glass. The normal electrical supply at an industrial site is a high voltage alternating three phase electrical supply which is normally connected to a transformer system to provide power at a desired voltage. Three phase electrical systems are commonly connected to a transformer by use of a star or delta configuration and the output from the secondary windings of such a transformer may be coupled in a closed delta arrangement. Such a system of supplying a three phase power supply using a closed delta system in an electric glass melter is shown in U.S. Pat. No. 4,607,372. In that patent specification electrodes are arranged in rows and a balanced electrical load is achieved on each of the three phases by using a strict geometric pattern for the electrode array. Using such a closed delta system for the three phase supply to the electrodes it is not possible to vary independently the power supplied through each of the different phases of the three phase supply. When the geometric arrangement of electrodes in the array is changed the power distribution in the melter will be varied which may lead to unbalanced loading. It is therefore necessary to maintain a predetermined geometric pattern for the electrode array which may limit variation of furnace design and operation. Furthermore it does not permit local variations in heat distribution if instability should occur during operation.
UK Patent 1319060 and U.S. Pat. No. 4,410,997, show other electrical heating systems used in glass melting where power is derived from a three phase electrical supply. They again require specific geometric symmetry of the electrode arrangements in order to maintain balanced electrical loads.
It is an object of the present invention to provide an electrical heating system for use in a glass melting furnace using a three phase AC supply wherein the electrical power supplied between selected pairs of electrodes may be adjusted to achieve a desired heat distribution.